It is well known that exhaust gases from internal combustion engines contain substances which are harmful to the environment and which can pose a threat to public health. For many years, a sustained effort has been made within the automotive industry to reduce the release to the atmosphere of harmful substances carried in exhaust gases, both by modifying the combustion process itself to give a reduced yield of harmful combustion products, and by treating the exhaust gases before their emission into the atmosphere, for example by providing a catalyst to induce chemical breakdown of the harmful constituents into benign compounds.
One class of harmful exhaust gas constituents comprises the oxides of nitrogen, with the generic chemical formula NOx, where x typically ranges from 0.5 to 2.5. Nitrogen oxides contribute to the formation of ground-level ozone, nitrate particles and nitrogen dioxide, all of which can cause respiratory problems. Furthermore, nitrogen oxides can lead to the formation of acid rain, and nitrous oxide (N2O) in particular is a greenhouse gas and contributes to the destruction of the ozone layer. It is therefore desirable to reduce the emission of nitrogen oxides into the atmosphere, and furthermore, new vehicles must comply with increasingly stringent limits on the acceptable levels of NOx emissions.
In certain circumstances, NOx emissions can be reduced by conventional exhaust gas catalysis, for example in a three-way catalyst comprising immobilised powders of platinum, palladium and rhodium. However, in diesel or lean-burn petrol combustion engines, a high concentration of oxygen is present in the exhaust gas, and this oxygen inhibits the catalysed breakdown of the nitrogen oxides in conventional systems. Consequently, a need has arisen for an alternative strategy to limit NOx emissions.
One strategy, known as selective catalytic reduction or SCR, involves the introduction of a reagent comprising a reducing agent, typically a liquid ammonia source such as an aqueous urea solution, into the exhaust gas stream. The reducing agent is injected into the exhaust gas upstream of an exhaust gas catalyst, known as an SCR catalyst, typically comprising a mixture of catalyst powders such as titanium oxide, vanadium oxide and tungsten oxide immobilised on a ceramic honeycomb structure. Nitrogen oxides in the exhaust gas undergo a catalysed reduction reaction with the ammonia source on the SCR catalyst, forming gaseous nitrogen and water. An example of such a system is described in International Patent Application No. WO 2004/111401 A.
Although aqueous urea is a convenient and cost-effective source of ammonia for SCR systems, the maximum temperature at which it can be used is somewhat limited. Urea crystals tend to precipitate when the temperature of the solution is greater than approximately 70° C. Precipitation is undesirable because the precipitates can cause blockages in the delivery system, for example in the small-diameter outlets typically provided in an atomising nozzle. In addition, the formation of precipitates alters the concentration of the remaining solution, so that the effective quantity of ammonia delivered to the exhaust flow becomes uncertain. This could lead to inefficient catalysis and an insufficient reduction in NOx emissions.
If aqueous urea is to be used effectively as a reagent in SCR, the system provided for dosing the exhaust gases with reagent should ideally be arranged to ensure that the temperature of the urea solution does not exceed the temperature at which precipitation occurs. However, the reagent must be discharged into the stream of hot exhaust gases, which are typically at a temperature of around 400° C. at the point where the reagent enters the exhaust gas stream. The reagent will therefore almost inevitably reach a temperature in excess of that at which solid precipitates begin to form.
In the Applicant's United States Patent Application No. US2004/0093856, a solenoid-operated reagent dosing pump is described. Because this pump can generate high reagent pressures, it is able to blow precipitates through an outlet nozzle. In this way, any solid particles that form due to overheating of the reagent can be forced out of the dosing system and into the exhaust gas stream and are prevented from blocking the flow of reagent. Furthermore, the use of a high-pressure solenoid pump allows the delivery of small quantities of reagent at high frequencies, with the result that the mixture of exhaust gas and reagent flowing on to the SCR catalyst has a more uniform temperature and composition in comparison to other systems, in which larger quantities of reagent are delivered at lower frequencies. This improves the efficiency of the reduction reactions occurring at the catalyst, because the temperature and gas composition can be better maintained at their optimum levels for reaction.
Although use of a solenoid pump offers significant advantages for reagent dosing devices, one potential drawback arises from the sensitivity of the solenoid to temperature. The efficiency of the solenoid, often expressed as the ratio of the mechanical power output to the electrical power input, decreases as the temperature of the solenoid increases. This decrease in efficiency is due in part to the increase in resistance of the coil with temperature. When used in a reagent dosing system, the solenoid tends to heat up due to the proximity of the pump to the hot exhaust system, and due to the resistive heating of the coil. The temperature in the vicinity of the solenoid is also relatively high, which that dissipation of heat from the solenoid into its surroundings is limited.
Against this background, it would be desirable to provide a reagent dosing device for use in an exhaust gas dosing system which overcomes or alleviates the abovementioned problems.